feat: decouple read FeedId from inbound MailboxId

Separate the two feed identities so the public read URL never reveals the
inbound address and vice-versa:

- FeedId becomes an opaque high-entropy token (read id + KV key); MailboxId
  (noun.noun.NN) owns the inbound address and the untrusted-input boundary
  via MailboxId.parse. They map only through the inbound:<mailbox> secondary
  index, resolved solely at reception.
- inbound index lifecycle is owned by FeedRepository: written by save/saveConfig,
  dropped by removeFromList(Bulk) — symmetric, never mirrored by hand (removes the
  manual delete in feed-service + the cron loop, and a silent empty-catch).
- Feed.mailboxId exposes a MailboxId VO (symmetry with Feed.id); the
  mailbox@domain shape lives on MailboxId.emailAddress(domain).
- Distinguish mailbox_unknown (no feed claims the address) from feed_not_found
  (dangling index) for observability; both forwardable, both 404.
- Drop the redundant EmailParser.extractMailbox pass-through so MailboxId.parse
  is the single parse boundary.

Docs (README/INSTALL/CLAUDE.md/landing) and tests updated; 439 tests green,
tsc clean, build dry-run OK.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/routes/admin/emails.tsx

@@ -169,7 +169,7 @@ emailsRouter.get("/feeds/:feedId/emails", async (c) => {
     return c.text("Feed not found", 404);
   }
 
-  const emailAddress = feedEmailAddress(feedId, env);
+  const emailAddress = feedEmailAddress(feedConfig.mailbox_id, env);
   const rssUrl = feedRssUrl(feedId, env);
   const atomUrl = feedAtomUrl(feedId, env);
 
@@ -466,6 +466,8 @@ emailsRouter.get("/emails/:emailKey", async (c) => {
   if (!emailData) return c.text("Email not found", 404);
 
   const feedId = repo.feedIdFromEmailKey(emailKey);
+  const feedConfig = await repo.getConfig(FeedId.unchecked(feedId));
+  if (!feedConfig) return c.text("Feed not found", 404);
   // Inline images render in place; only downloadable attachments go in the list.
   const attachments = (emailData.attachments ?? []).filter((a) => !a.inline);
 
@@ -584,7 +586,10 @@ emailsRouter.get("/emails/:emailKey", async (c) => {
                 value={new Date(emailData.receivedAt).toLocaleString()}
               />
               <SenderField from={emailData.from} feedId={feedId} />
-              <CopyField label="To:" value={feedEmailAddress(feedId, env)} />
+              <CopyField
+                label="To:"
+                value={feedEmailAddress(feedConfig.mailbox_id, env)}
+              />
             </div>
           </div>